1. Field of the Invention
The present invention relates to an electric power steering device for motor vehicles. More particularly, the present invention relates to an electric power steering device that can detect faults to occur in the motor current detecting means.
2. Prior Art
An electric power steering device employed for a motor vehicle detects a steering torque generated at a steering shaft by an operation of the steering wheel and a speed of the motor vehicle and drives the motor according to detected signals, thereby assisting the steering power of the steering wheel. An electronic control circuit is used to control such an electric power steering device as follows: a value of a current to be supplied to the motor is calculated based on the steering torque detected by a torque sensor and the vehicle speed detected by a vehicle speed sensor and the supply current is controlled based on the calculation result.
Concretely, the electronic control circuit controls the supply current so that a large assist steering power is supplied to the steering wheel when the steering torque is generated by an operation of the steering wheel and the detected vehicle speed is zero or low, and a small assist steering power is supplied to the steering wheel when the speed of the motor vehicle is high, thereby optimizing the supply of the assist steering power in accordance with the running state of the motor vehicle.
In such an electric power steering device, the actual current that flows in the motor is fed back and controlled so that the current matches with the target value calculated based on the steering torque and the vehicle speed. The electric power steering device is thus provided with a motor current detecting means for detecting the current that flows in the motor.
In such an electric power steering device, if the motor current detecting means breaks down, accurate motor current measurement is disabled and accordingly, an excessive current flows in the motor. As a result, an excessive assist steering power is supplied to the steering wheel or a sufficient current is not supplied to the motor. The assist steering power to be supplied to the steering wheel will thus become insufficient.
Furthermore, an operation check is usually done for the controlling device of the motor vehicle at the engine start-up time. An operation check is also done for the motor current detecting means at this time. And, when a current is supplied to the motor in the operation check, the motor rotates. If the motor shaft is coupled with the steering mechanism at this time, the steering wheel also rotates, thereby an unexpected accident might occur.
To avoid such an accident, Japanese Patent Laid Open Publication No. H8-91239 (91239/1996) proposes the use of a fault determining means. According to the invention, a fault to occur in the motor current detecting means is determined based on a current value expected when a voltage is applied to the motor only for a short time assumed to be larger than the electrical time constant and smaller than the mechanical time constant of the motor, and a motor current detected by the motor current detecting means itself.
The fault determining means of the above-described motor current detecting means determines a fault based on a voltage applied to the motor only for a short time just after the engine is started up by turning on the ignition key, that is, only for a time whose value is larger enough than the electrical time constant and smaller enough than the mechanical time constant of the motor. This is needed to prevent the above described unexpected accident to be caused by an unexpected rotation of the steering wheel when the motor begins rotating just after the engine starts.
The motor, when it is kept used for a certain time, causes an electrically insulated oxide film to be formed on contact surfaces between the commutator and the brush of the motor. The oxide film becomes thicker with time, thereby the electric resistance between the contact surfaces rises. To apply a higher voltage is thus required to rotate the motor in this connection.
FIGS. 9(a) and 9(b) are diagrams showing the disturbance by such an oxide film against motor current measurement. As to be understood from FIG. 9(a), a line A denotes the normal state of the motor, in which no oxide film is formed on the contact surface, since the motor is new. The applied voltage and current of the motor are in a proportional relationship with each other. The motor current increases in proportion to the rising of the applied voltage. Another line B shows a case in which an oxide film is formed on the contact surfaces. The motor current does not increase in proportion to the rising of the applied voltage in this case. When the applied voltage reaches the value S, however, the oxide film causes breakdown (puncture), thereby the electric resistance of the film drops sharply. Consequently, a current corresponding to the normal voltage comes to flow in the motor.
FIG. 9(b) shows how the applied voltage that causes breakdown of the oxide film rises. When the oxide film becomes thicker with time, applied voltage that causes breakdown of the oxide film will be raised up S1, S2, S3 and S4 with time.
As described above, application of a low voltage to the motor only for a short time might cause a problem in determination of a fault in the motor current detecting means since the motor current is not detected or only a few motor current is detected due to the oxide film formed on the contact surface. It might thus be determined wrongly that the motor current detecting means is defective.